Bending backwards can cause a herniated disc. It depends on the position of the head.

When one bends backwards, the virtual line of the center of gravity of the head should be aligned with the spine. When the backward movement of the head overshoots the virtual line of the head’s center of gravity, such as in a backlash, the spinal discs will be subject to sudden compression. This causes herniation of a disc.

Although backlash occurs suddenly, much slower bending backwards can also shift disc positions.